Graduate Thesis Or Dissertation


Permineralized Mesozoic Moss Gametophytes and their Implications for Bryophyte Evolution Public Deposited

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  • Mosses (Phylum Bryophyta) are widely considered to be among the most ancient groups of land plants (embryophytes) and they are the second most speciose embryophyte phylum with ~13,000 extant species. Despite their diversity and antiquity, mosses have a limited fossil record, which primarily consists of gametophytes entombed in Cenozoic amber. However, surveys of Mesozoic floras from North and South America have recently uncovered a wealth of permineralized moss gametophytes. These fossils present the first opportunity to explore macroevolutionary trends in mosses over deep geologic time, with rich sampling of pre-Cenozoic diversity. The over-arching goal of this dissertation is to explore moss evolution over geologic time, using these recently discovered permineralized gametophytes. There are four main objectives: 1) document moss diversity in Late Jurassic and Cretaceous deposits by characterizing anatomically preserved fossil mosses, 2) investigate patterns in moss biogeography using data from the fossil record, 3) understand the longevity of moss genera over geologic time, and 4) explore the hypothesis that modern moss lineages diversified as part of the Cretaceous Terrestrial Revolution using paleontological data. Herein, six new taxa are characterized based on permineralized gametophytes from one Late Jurassic and three Cretaceous deposits. All fossils were studied using light microscopy. In Chapter 2, a permineralized acrocarpous moss gametophyte from the Late Jurassic of Patagonia, Argentina, is described. This fossil was prepared using the petrographic thin sectioning technique. The gametophyte shoot is unbranched, and leaves have a broad sheathing leaf base that narrows distally, forming a free leaf blade. Costa is strong, with an adaxial epidermis of bulging-mammillose cells, subtended by guide cells and a single abaxial stereid band. An abaxial epidermis is lacking. In the free leaf blade, the lamina is bistratose, with an adaxial layer of bulging-mammillose cells. This fossil gametophyte displays a novel combination of characters that suggest basal affinities within the families Timmiellaceae or Polytrichaceae. Both of these families are members of a grade within Subphylum Bryophytina basal to Subclass Bryidae. Therefore, this fossil is described as a new genus and species assignable to Subphylum Bryophytina, Heinrichsiella patagonica gen. et sp. nov. Heinrichsiella is the oldest evidence of this basal grade within Subphylum Bryophytina by at least 15 Ma. In Chapter 3, a permineralized moss gametophyte assignable to the extant genus Cynodontium is described from the Late Cretaceous of the North Slope of Alaska. This fossil was permineralized in a terrestrial limestone and prepared using the cellulose acetate peel technique. The moss gametophyte branches irregularly and has leaves with a broad base, narrow blade, and excurrent costa. Leaf margins are narrowly recurved basally, becoming bistratose and denticulate distally. Costa is strong and composed of a poorly defined epidermis, guide cells, and thin-walled substereids. Leaf blade is keeled basally, becoming channeled distally. Laminar cells are subquadrate or rectangular proximally, becoming quadrate distally. This fossil has a combination of gametophytic characters diagnostic of the extant genus Cynodontium, but displays a novel combination of characters that are distinct from any extant species of that genus. Therefore, this fossil is described as Cynodontium luthii sp. nov. This species is the oldest evidence for Cynodontium by at least 15 Ma and reveals that some genera found in the High Arctic today also lived in the region during the Late Cretaceous, when it experienced a temperate climate. In Chapter 4, three new species of the extant genus Sphagnum are described based on permineralized gametophytes from the Late Santonian to Campanian of Vancouver Island, Canada. These fossils were permineralized in near-shore marine calcareous concretions and prepared using the cellulose acetate peel technique. All three species have unistratose leaves that are composed of smaller cells (chlorocysts) that alternate with larger, porose cells (hyalocysts). Collectively, these features are diagnostic of Sphagnum. The three species are most easily distinguished based on their stem anatomy. Stems of Sphagnum species one have a unistratose hyalodermis, while stems of Sphagnum species two have a multistratose hyalodermis, and stems of Sphagnum species three completely lack a hyalodermis. Taxonomic comparisons with extant species of Sphagnum are difficult due to the fragmentary nature of the specimens. Sphagnum species one and two have combinations of features found in several extant sections of Sphagnum. However, the absence of a hyalodermis is a character that is only found in some species of Sphagnum Section Cuspidata. Therefore, Sphagnum species three can be assigned to Section Cuspidata and is the only pre-Pleistocene evidence for that section. These fossils also represent the first reports of permineralized Sphagnum gametophytes and reveal that Sphagnum was anatomically diverse by the Late Cretaceous, which is far earlier than predicted by some molecular dating analyses. In Chapter 5, permineralized moss gametophytes assignable to the family Leucobryaceae are described from the Valanginian (~136 Ma) of Vancouver Island, Canada. These fossils were permineralized in near-shore marine calcareous concretions and prepared using the cellulose acetate peel technique. The gametophyte shoots are irregularly branched, leaves are almost entirely comprised of an expanded costa. Lamina is restricted to the sheathing leaf base and composed of small-diameter porose cells that are isodiametric in cross section. Anatomically, the costa is comprised of larger porose cells that are rectangular in cross section (leucocysts) and much smaller, thicker-walled cells, which are triangular in cross section for most of the leaf (chlorocysts). Costa is typically homostrosic, with a single layer of leucocysts on either side of centric chlorocysts. The unusual leaf anatomy of these fossils is diagnostic of the leucobryoid clade within the family Leucobryaceae. However, the fossils display a mosaic of characters, combining features of two leucobryoid genera (Leucobryum and Steyermarkiella). Therefore, they will be assigned to a new genus and species. These fossils are the oldest evidence of the family Leucobryaceae by at least 88 Ma and the oldest evidence of the leucobryoid clade within this family by >115 Ma. In Chapter 6, the role of paleontological data in bryophyte systematics is summarized based on a comprehensive review of the paleobryological literature, considering techniques for integrating fossils into evolutionary analyses with extant plants as well as aspects of bryophyte structure and ecology relevant to taphonomy. Bryophytes are unusual in that the group has a deep evolutionary history, but a sparse fossil record. The paucity of bryophyte fossils is driven by phenotypic and ecological features related to the small size and patchy ecological distributions of most members of the group as well as incomplete exploration of the fossil record. However, fossils provide unique insight on the evolution of organisms (including bryophytes), over deep time. They are the only means to independently test hypotheses about evolution generated from extant bryophytes and are essential for making accurate inferences regarding speciation and extinction rates, patterns of morphological evolution, and biogeography. The review concludes that development of robust morphological datasets for bryophytes, that would allow fossils to be integrated into macroevolutionary analyses alongside extant plants as well as focused efforts to uncover and describe pre-Cenozoic bryofloras are essential for bringing deep-time evolutionary perspectives to bryophyte systematics.
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